This patent application claims priority to Japanese Patent Document No. P2001-056330 filed on Mar. 1, 2001, the disclosure of which is incorporated herein by reference.
The present invention relates to a manufacturing method and a manufacturing apparatus of a carbonaceous material. More particularly, for the present invention relates to a manufacturing method and apparatus for single-walled carbon nanotubes or other carbonaceous materials by using arc discharge.
Carbon nanotubes are new materials first reported by S. Iijima in Nature, Vol. 354 (1991) 56 in 1991. Especially, single-walled carbon nanotubes (SWNT) have been figured out theoretically to change in electronic physicality from a metallic nature to a semiconductive nature, depending upon the way of winding of its helix, i.e. so-called chirality, and it is remarked as a hopeful electronic material of the next generation. Actually, there are various proposals of its applications to nanoelectronics materials, field electron emission emitters, highly directional radiation sources, soft X-ray sources, one-dimensional conduction materials, high-heat conduction materials, hydrogen storage materials, and others. Additionally, binding functional groups on surfaces, metal coating or containment of foreign substances will further expand the range of application of carbon nanotubes.
As a method of manufacturing single-walled carbon nanotubes and other carbonaceous materials, it has been proposed to compound a large mass of such materials by a so-called arc discharge process making use of arc discharge from a carbon rod as an electrode. This method produces carbonaceous materials by generating arc discharge in an arc discharge portion composed of juxtaposed anode and cathode.
An example of manufacturing apparatus of carbonaceous materials relying on an arc discharge process is shown in FIG. 1. The manufacturing apparatus 101 includes a cylindrical reaction tube 111 in which an anode 113 and a cathode 114 are juxtaposed via a light distance. The anode 113 is electrically connected to a positive-pole current inlet terminal 142, and the cathode 114 is electrically connected to a negative-pole current inlet terminal 141. These two current inlet terminals 141, 142 are electrically connected to a current supply portion 112 located outside the reaction tube 111 such that a voltage can be applied to the anode 113 and the cathode 114. The arc discharge portion is defined by distal ends where the anode 113 and the cathode 114 are opposed. The arc discharge portion is located approximately in the center of the reaction tube 111 in its axial direction, and an electric furnace 124 for heating the arc discharge portion is provided outside the portion of the reaction tube 111 aligned with the art discharge portion.
The anode 113 is a carbon electrode made of carbon added with a metal functioning as a catalyst such as iron, cobalt, nickel, lanthanum, or the like. The catalyst is used upon manufacturing carbonaceous materials such as single-walled carbon nanotubes by arc discharge. The cathode 114 is a pure carbon electrode containing no catalyst.
Caps 111C, 111D covering end portions of the reaction tube 111 are provided at opposite ends of the reaction tube 111 to be able to seal the interior of the reaction tube 111 from the atmospheric air. The cap 111C have a through hole 111a that penetrates it in the axial direction and permits communication between the interior and the exterior of the reaction tube 111. Connected to the through hole 111a is an inactive gas injector 143 via a hose 117. The inactive gas injector 143 can supply inactive gas such as He or Ar into the reaction tube 111. A flowmeter 118 is interposed in the hose 117 such that the velocity of the inactive gas injected into the reaction tube 111 can be changed.
The cap 111D has a through hole 111b penetrating it in the radial direction from its circumferential surface to permit communication between the interior and the exterior of the reaction tube 111. A pump 121 is connected to this through hole 111b via a hose 119. The pump 121 can discharge gas inside the reaction tube 111 to the exterior thereof by making use of a reduced pressure. A flowmeter 120 is interposed in the hose 119 such that the velocity of inactive gas, or the like, discharged from inside the reaction tube 111.
The cap 111D has another through hole 111c that penetrates it in the axial direction, and receives a double tube 122 penetrating and extending beyond it. Therefore, part of the double tube 122 resides in the reaction tube 111. On one of opposite end portions of the double tube 122 residing in the double tube 122, a capturer 123 for capturing carbonaceous materials produced in the arc discharge portion is mounted. The capturer 123 defined therein a space communicating with a space defined by the inner periphery of an outer tube and the outer periphery of an inner tube of the double tube 122, and a space communicating with a space defined by the inner periphery of the inner tube of the double tube 122. These two spaces are communicating with each other. In this configuration, when cooled water is supplied to the space defined by the inner periphery of the inner tube from one end of the double tube 122 opposite from the said end having the capturer 123, the cooled water passes through the space defined by the inner periphery of the inner tube, then reaches the inside of the capturer 123, cools the capturer 123 there, thereafter flows into the space defined by the inner periphery of the outer tube of the double tube 122 and the outer periphery of the inner tube thereof, and exits from the other end of the double tube 122.
Next explained is a method of manufacturing carbonaceous materials like single-walled carbon nanotubes. The anode 113 is made by crushing carbon to powder, then preparing a mixture of the powder carbon and powder of a catalyst such as iron, nickel, cobalt or lanthanum, shaping the mixture into the form of the anode 113, and sintering and/or machining it. The cathode 114 is made by directly shaping carbon into the form of the cathode 114. After that, the anode 113 and the cathode 114 are set in a carbonaceous material manufacturing apparatus 101, and the interior of the reaction tube 111 is once evacuated to a vacuum. After that, under the condition where an inactive gas injector 143 supplies inactive gas into the reaction tube and the pump 121 discharges the inactive gas from the reaction tube 111, that is, under the condition where a gas flow is made in the arc discharge portion, arc discharge is executed to produce a carbonaceous material such as single-walled carbon nanotubes from the carbon composing the anode 113 by catalysis of the catalyst. More specifically, in the arc discharge portion, metal and carbon simultaneously vaporize from the anode 113, and the vaporizing carbon appears as soot. The soot obtained contains graphite, amorphous carbon, catalytic metal, oxides of the catalytic metal, and others in addition to single-walled nanotubes. The soot containing carbonaceous materials such as single-walled carbon nanotubes produced in the arc reaction portion is transported to the capturer 123 located downstream by the flow of the supplied inactive gas.
In order to increase the recovery percentage of single-walled carbon nanotubes and other carbonaceous materials produced by the above-explained arc discharge method, various techniques have been disclosed heretofore.
According to Japanese Patent Laid-Open Publications Nos. hei 6-157016 and hei 6-280116, it is appreciated that the recovery percentage of single-walled carbon nanotubes by the arc discharge method largely depends upon the partial pressure of gas in the reaction tube where the single-walled carbon nanotubes are produced. Japanese Patent Laid-Open Publication No. hei 6-280116 discloses that the recovery percentage of single-walled nanotubes can be increased by maintaining the pressure of the inactive gas in the reaction tube in a range not lower than 200 Torr (about 26.7 kPa). Japanese Patent Laid-Open Publication No. hei 6-157016 discloses that the recovery percentage of single-walled carbon nanotubes can be optimized when the partial pressure of inactive gas in the reaction tube is in the range of 500 to 2500 Torr (approximately 66.7 to 333.3 kPa). Furthermore, Japanese Patent Laid-Open Publication No. hei 6-157016 discloses that it is possible to increase the recovery percentage of single-walled carbon nanotubes by adjusting the temperature of the arc discharge portion in the range of 1000xc2x0 C. to 4000xc2x0 C.
When single-walled carbon nanotubes are produced by the arc discharge method, carbonaceous materials adhere on the wall surface of the reaction tube 111 as a sootlike product or a web-like product. Single-walled carbon nanotubes are contained much more in the web-like product. Sootlike products are considered to mainly comprise amorphous carbon. Taking it into consideration, in order to obtain products rich in single-walled carbon nanotubes and thereby increase the recovery percentage, efficient recovery of web-like products is important.
Japanese Patent Laid-Open Publication No. hei 8-12310 discloses a method of efficiently recovering web-like products containing soot generated by arc discharge. This method obtains single-walled carbon nanotubes of high purity to an extent by recovering soot-contained web-like products from inner wall surfaces of a reaction tube and thereafter purifying the products by an acidic solution or thermal oxidation.
T. Sugai et al. in Jpn. J. Appl. Phys. Vol. 38 (1999) L477 and T. Sugai et al., in J. Chem. Phys. Vol. 112, (2000) 6000 report a method of obtaining a high recovery percentage of single-walled carbon nanotubes by arc discharge in an electric furnace. It is reported there that single-walled carbon nanotubes were evaluated by electronic microscopy or Raman spectroscopy and that more efficient recovery of single-walled carbon nanotubes than that by arc discharge in a reaction tube was confirmed.
In contrast, the methods of manufacturing carbonaceous materials disclosed by Japanese Patent Laid-Open Publications Nos. hei 6-157016 and hei 6-280116 do not include a process of purifying soot or other carbonaceous materials obtained, and samples obtained still contain a significant quantity of metal catalyst and amorphous carbon. Therefore, in order to increase the purity of single-walled carbon nanotubes, purification of obtained carbonaceous materials will be indispensable. Japanese Patent Laid-Open Publication No. hei 8-12310 certainly includes purification of obtained carbonaceous materials, but after removing them from the reaction tube and exposing them to the atmospheric air. Since metal catalyst is in form of fine particles, as soon as it is exposed to the atmospheric air, its surface is oxidized. Metal catalyst, once oxidized, is difficult to remove. Therefore, it has been difficult to obtain high-purity single-walled carbon nanotubes substantially free from catalyst.
The present invention provides improved methods and apparatuses for manufacturing carbonaceous materials, with which high-purity carbon-based materials, such as single-walled carbon nanotubes can be obtained efficiently.
In an embodiment, the present invention provides a carbonaceous material manufacturing method in which an anode made of a carbon-based material and a cathode made of a carbon-based material and opposed to the anode are placed to define an arc discharge portion between the anode and the cathode in a reaction tube defining a carbonaceous material generating chamber, the arc discharge portion producing arc to generate carbonaceous materials when a voltage is applied across the anode and the cathode while the arc discharge portion is exposed to an atmospheric gas, and the atmospheric gas being supplied to flow in a predetermined direction enabling the atmospheric gas to pass through the discharge portion in the reaction tube. The method includes recovering the generated carbonaceous materials in a carbonaceous material capturer located downstream of the arc discharge portion with respect to the flowing direction of the atmospheric gas while heating the carbonaceous material capturer.
The atmospheric gas, in an embodiment, is preferably a catalytic gas.
The anode, in an embodiment, is preferably made of a carbon-family or carbon-based material not containing a catalyst.
Alternatively, the atmospheric gas, in an embodiment, is preferably a mixture gas of an organic gas and a catalytic gas.
Alternatively, the anode is, in an embodiment, preferably made of a carbon-family gas not containing a catalyst.
Alternatively, the atmospheric gas, in an embodiment, is preferably an organic gas.
The heating of the carbonaceous material capturer is preferably carried out under a reduced pressure in an embodiment.
The art discharge and the heating of the carbonaceous material capturer are preferably carried out simultaneously.
During the arc discharge, the atmospheric gas preferably makes a helical flow traveling around the arc discharge portion along the line connecting the anode and the cathode.
Preferably, different kinds or types of atmospheric gases are independently supplied into the reaction tube and mixed therein to make a helical flow of the mixed gas.
The invention further provides a carbonaceous material manufacturing apparatus. The apparatus includes:
a reaction tube defining a carbonaceous material generating chamber;
an anode made of a carbonaceous material and placed in the reaction tube;
a cathode made of a carbonaceous material and opposed to the anode to define an arc discharge portion between the anode and the cathode in the reaction tube; and
a current supply portion connected to the anode and the cathode to induce arc discharge between the anode and the cathode,
wherein an atmospheric gas supply portion is connected in communication with the reaction tube to supply the atmospheric gas to flow in a predetermined direction toward the arc discharge portion,
wherein a carbonaceous material capturer is located downstream of the arc discharge portion in the reaction tube with respect to the flowing direction of the atmospheric gas, and
wherein the apparatus includes a heater located inside or outside the carbonaceous material capturer to heat the carbonaceous material capturer.
The reaction tube, in an embodiment, preferably has an inner diameter small enough to limit the flow of the atmospheric gas to one direction and prevent its convection in the reaction tube.
The atmospheric gas, in an embodiment, is preferably a mixture gas of an organic gas and a catalytic gas.
The anode, in an embodiment, is preferably made of a carbon-family material not containing catalyst.
Alternatively, the atmospheric gas, in an embodiment, is preferably a catalytic gas.
The anode, in an embodiment, is preferably made of a carbon-family material not containing a catalyst.
Alternatively, the atmospheric gas, in an embodiment, is preferably an organic gas.
Preferably, the reaction tube, in an embodiment, is approximately elliptic in cross section; the atmospheric gas supply portion has a gas supply tube connected to the reaction tube to supply gas toward the arc discharge portion from an upstream position thereof to ensure carbonaceous materials generated by arc discharge in the arc discharge portion to be transported toward the capturer; and the gas supply tube extends in an approximately tangential direction of the reaction tube to generate a helical flow in the reaction tube.
The gas supply tube preferably includes, in an embodiment, at least two tubes, i.e. a first tube connected in communication in an approximately tangential direction of the reaction tube to supply first gas into the reaction tube, and a second tube connected in communication at a location different from that of the first tube in an approximately tangential direction of the reaction tube to supply second gas into the reaction tube.
Preferably, a first flowmeter for permitting the first gas to flow in the first tube at a first velocity is connected to the first tube, and a second flowmeter for permitting the second gas to flow in the second tube at a second velocity different from the fist velocity is connected to the second tube, in an embodiment.
The first gas is preferably an organic gas in an embodiment.
The second gas is preferably a catalytic gas in an embodiment.
Preferably, the gas supply tube extends with an acute angle from the arc discharge portion toward the capturer and is connected to the reaction tube in an embodiment.
Preferably, an inner tube smaller in diameter than the reaction tube is coaxially disposed in the reaction tube to reside at least at the position where the gas supply tube is connected in an embodiment.
The reaction tube preferably has a thinner portion around the arc discharge portion, which has an inner circumferential cross-sectional area larger than the inner circumferential cross-sectional area of the remainder portion of the reaction tube in an embodiment.
Preferably, the thinner portion extends up to slightly before the capturer, and the reaction tube is enlarged in its diameter immediately before the capturer in an embodiment.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.